The present invention relates to a color cathode ray tube with a panel having a substantially flat outer surface, and more particularly, to a color cathode ray tube improved in visibility and in strength for the maintenance of a curved surface of a shadow mask.
Generally, a color cathode ray tube has a vacuum envelope that is composed of a glass panel and a glass funnel. In the color cathode ray tube, three electron beams emitted from an electron gun assembly in a neck of the funnel are deflected by a magnetic field that is generated by a deflection yoke attached to the outside of the funnel. A phosphor screen, which is formed of three-color phosphor layers on the inner surface of an effective portion of the panel, is scanned horizontally and vertically with the electron beams transmitted through the shadow mask, whereupon a color image is displayed on the screen.
In the color cathode ray tube of this type, in general, a side wall or skirt portion is attached to the peripheral portion of a faceplate that has a substantially rectangular effective portion. In order to obtain strength high enough to stand the load of the atmospheric pressure that acts on the vacuum envelope, the panel is formed having different curved surfaces as its inner and outer surfaces so that the central portion of the effective portion is thinner than the peripheral portion. Moreover, the outer surface of the effective portion is a curved surface such that its height above sealed surfaces of the panel and the funnel is greatest in the central portion and is reduced with distance from the central portion. More specifically, the outer surface of the effective portion is a spherical surface, a cylindrical surface having an infinite curvature radius in the minor-axis direction and a curvature in the major-axis direction, or a curved surface that can be represented by a high-order polynominal expression.
Flattening the outer surface of the effective portion of the panel has recently been promoted to improve the visibility of images. Described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685 is an example of an improved panel in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction.
The visibility of an image formed on the phosphor screen is influenced by the shape of the inner surface of the effective portion, which is fitted with the screen, as well as by the shape of the outer surface of the effective portion. In the case where the outer and inner surfaces of the effective portion are different, the difference in thickness between the central and peripheral portions of the effective portion has no substantial influence on the visibility if it is small. If the outer surface is flattened so that the difference in thickness between the central and peripheral portions increases, however, the influence of the difference in thickness is remarkable.
In order to improve the visibility, both the inner and outer surfaces of the effective portion should preferably be flat. In the case of a panel, such as the one described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685, in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction, however, the curvature of the short-side inner surface of the effective portion sometimes may be seen sideways from the short sides, although the effective portion frontally looks rectangular. Accordingly, the panel is expected to be further improved for the enhancement of the visibility or the display of flat images, which is a primary object of the flattening of the outer surface of the effective portion.
In the process of molding this glass panel, moreover, the cooling speed of glass varies due to the great difference in thickness between the central and peripheral portions, so that accurate curved surfaces cannot be obtained, in some cases.
On the other hand, the shadow mask is composed of a substantially rectangular mask body with a thickness of about 0.1 to 0.3 mm and a substantially rectangular mask frame attached to the peripheral portion of the mask body. A large number of beam apertures are formed in an effective surface of the mask body that faces the phosphor screen on the inner surface of the effective portion of the panel.
In general, the effective surface of the mask body must sustain a predetermined relation to the inner surface of the effective portion of the panel, so that these surfaces are similar curved surfaces. The shadow mask of the conventional color cathode ray tube has a curved surface whose profile is highest in the central portion and is reduced with distance from the central portion when the panel is turned upward. More specifically, the curved surface of the shadow mask, like that of the panel, is a spherical surface, a cylindrical surface having an infinite curvature radius in the minor-axis direction and a curvature in the major-axis direction, or a curved surface that can be represented by a high-order polynominal expression.
In the case of the panel in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature in the minor-axis direction, the effective surface of the mask body of the corresponding shadow mask, like the inner surface of the effective portion of the panel, must be a cylindrical surface having a substantially infinite curvature radius in the major-axis direction and a curvature radius in the minor-axis direction. One such shadow mask is described in Jpn. Pat. Appln. No. 9-3632.
In general, the strength for the maintenance of the curved surface of the shadow mask is settled depending on the shape of the curved surface of the effective surface of the mask body, the thickness of the mask body, and the configuration, size, and arrangement of the electron beam apertures. If the thickness of the mask body and the configuration, size, and arrangement of the electron beam apertures are fixed, therefore, the curved surface maintenance strength of the shadow mask is settled depending on the shape of the curved surface of the effective surface.
If the maximum and minimum curvature radii, among curvature radii in all directions with respect to an optional point on the effective surface, are Rmax and Rmin, respectively, as indexes for the curved surface maintenance strength of the shadow mask, there is an average curvature that can be defined by the sum (1/Rmax+1/Rmin) of the minimum and maximum curvatures 1/Rmax and 1/Rmin, the respective reciprocals of the maximum and minimum curvature radii.
If the depth of depression (difference in distance along the tube axis between the center and each diagonal portion of the mask) of each diagonal portion of the effective surface of the conventional shadow mask at which the depression has its maximum is fixed, the curved surface maintenance strength can be enhanced by forming the effective surface in the shape of a cylindrical surface that has the highest average curvature, as in the case of the shadow mask described in Jpn. Pat. Appln. No. 9-3632.
In a shadow mask that is combined with a panel in which the outer surface of the effective portion is a substantially flat surface, however, the depth of depression of each diagonal portion of the effective surface is so small that the curved surface maintenance strength cannot be great enough.
The reduced curved surface maintenance strength of the shadow mask has various bad influences on the performance of the color cathode ray tube.
If any external force or impact is applied to the color cathode ray tube, the curved effective surface of the shadow mask is easily deformed, so that the image quality is lowered. If the color cathode ray tube is subjected to vibration, moreover, it is liable to resonate and undergoes lowering of the color purity (i.e., howling). If the shadow mask undergoes local thermal expansion attributable to collision of high-density electron beams in locally displaying bright images by means of the high-density electron beams, the color purity is lowered by the resulting local doming. The lowering of the color purity attributable to the local doming advances to the highest degree in intermediate regions 2 of a picture 1 a little nearer to short sides S0 than to the center, in the major-axis direction (i.e., X-axis direction), as shown in FIG. 1.
In general, the local doming is believed to be able to be effectively restrained by applying residual stress to the effective surface of the mask body. In connections with this, the curved surface maintenance strength of the shadow mask serves to restrain thermal expansion of the shadow mask. The reduction of the curved surface maintenance strength of the shadow mask, especially the effective surface, on the major axis accelerates the lowering of the color purity attributable to the local doming.
As mentioned before, flattening the color cathode ray tube has recently been promoted to improve the visibility of images. A panel has been proposed in which the outer surface of the effective portion is a substantially flat surface and the inner surface is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature radius in the minor-axis direction.
In the panel of this type, however, there is a great difference in thickness between the central and peripheral portions of the effective portion, so that the curvature of the short-side inner surface of the effective portion sometimes may be seen sideways from the short sides. Thus, there is room for further improvement of the panel in view of the display of flat images, and desired visibility cannot be obtained. If the difference in thickness is too great in the process of molding the panel, moreover, the cooling speed of glass varies, so that accurate curved surfaces cannot be obtained, in some cases.
On the other hand, a shadow mask corresponding to the panel has been proposed in which the effective surface of the mask body is a substantially cylindrical curved surface having a substantially infinite curvature radius in the major-axis direction and a curvature radius in the minor-axis direction.
In the shadow mask of this type, however, the depth of depression of each diagonal portion of the effective surface is so small that the curved surface maintenance strength cannot be great enough. If any external force or impact is applied to the color cathode ray tube, therefore, the curved effective surface of the shadow mask is easily deformed, so that the image quality is lowered. If the color cathode ray tube is subjected to vibration, moreover, it is liable to resonate and undergoes lowering of the color purity. In locally displaying bright images by means of high-density electron beams, moreover, the shadow mask undergoes thermal expansion attributable to collision of the high-density electron beams, and the color purity is lowered by local doming. Further, the mask body is liable to suffer side cutout or the like, so that it is difficult to obtain a curved surface with a predetermined curvature radius.